The most commonly used and most well known air separation process for oxygen production is the Linde double column cycle invented in the first half of the century. The basic concept of the Linde double column cycle is to have thermal communication between the top of the higher pressure column and the bottom of the lower pressure column to condense the vapor nitrogen from the higher pressure column and reboil the liquid oxygen in the bottom of the lower pressure column. A portion of the liquid nitrogen that is taken out of the higher pressure column is then sent to the top of the lower pressure column as the reflux. Such an air separation plant can recover more than 90% of the oxygen in the feed air, so that vapor coming out of the lower pressure contains more than 97% nitrogen. In cases in which large quantities of nitrogen is demanded as coproduct, and the nitrogen has to meet a certain purity requirement, a waste stream is taken out a few trays below the top of the lower pressure column in order to control the nitrogen product purity. Such waste streams, however, are still designed to contain more than 95% nitrogen so that the recovery of oxygen, and that of argon can be kept high. Flow of such a waste stream is also usually limited to below 15%, which is enough for regeneration of the mole sieve adsorption bed using thermal swing adsorption-desorption technique.
When liquid is also produced in substantial quantities, the conventional method is to introduce a refrigeration system in which nitrogen is used as the working fluid. This system produces liquid nitrogen which is used as product and/or additional reflux for the air separation unit which, still keeps the Linde double column with characteristics described above, such as can be seen in U.S. Pat. No. 3,605,422. When the liquid/feed ratio is relatively small, a refrigeration system in which air is used as the working fluid can be used. Such a liquefier uses the refrigeration from expansion of a portion of the high pressure air to condense another portion of high pressure air. The air separation unit, however, is still the Linde double column cycle with characteristics described before, such as is shown by U.S. Pat. No. 4,152,130.
Since the above mentioned processes all use the conventional Linde double column cycle, which achieves an essentially complete separation of air into oxygen and nitrogen (and argon in some applications), they are appropriate if almost all of the products of air separation, i.e. oxygen and nitrogen (and argon) are required. In many cases, however, a large portion of the nitrogen produced from an air separation plant cannot find use (other than for chilling water in a waste tower). Accordingly, some of the product nitrogen is vented to atmosphere after it exits the cold box. In other cases, some of the product gas is demanded as liquid products. In either of these cases, better cycles can be used to reduce the power consumption as well as capital cost of the air separation unit.
U.S. Pat. No. 5,165,245 discloses a process with an elevated pressure double column system. In the process, refrigeration from expansion of the high pressure nitrogen is used to produce liquid products. The benefits of such elevated pressure processes include reduced pressure drop loss and reduced sized process equipment, e.g., pipes and heat exchangers. Unfortunately, if no liquid products are produced or needed, then such a process is not suitable.